Copper process or aluminum process is the main step for forming interconnects (wirings) in an integrated circuit (IC). For copper, its resistivity (ρCu=1.7 μΩ-cm) is 40% lower than that of aluminum (ρA1=2.7 μΩ-cm) and 50% lower than that of copper and aluminum alloys (ρAl=Cu(0.5 at %)=2.9˜3.3 μΩ-cm). Therefore, copper wirings used as interconnects in ICs can effectively reduce RC delay. In addition, copper has better electromigration and stress-void resistance than aluminum. Comparing with the aluminum process, copper process has an advantage in cost.
However, copper wirings have below defects. Copper is not able to form fluorides and chlorides under 200-300° C. so that plasma etching is not easily processed. Besides, copper can not form a dense and passivated protective oxide layer over its surface. Copper atoms will fast spread into SiO2 by heat or bias diffusion and build up deep impurity levels. Adhesion between copper and SiO2 is not as good as that between aluminum and SiO2. Most of all, copper easily reacts with silicon so as to produce Cu—Si compounds. This changes the designed characteristics of the ICs and causes worse performance. Hence, many interconnects of ICs are still made of aluminum wiring.
However, aluminum has low adhesive ability to connect with other metals. Many methods are provided for forming an electrical interconnection between the aluminum wiring and a copper bonding pad. A commonly used means is illustrated through FIG. 1A to FIG. 1C. A semiconductor wafer 1 of an IC has an aluminum wiring 2 formed on one surface. Then, a passivation layer 3 is formed to cover the semiconductor wafer 1 and the aluminum wiring 2. An etching process is carried on to expose the aluminum wiring 2. Finally, a copper pad 4 is formed on the aluminum wiring 2 and partial semiconductor wafer 1.
Conventionally, in a fab, the etching process can be any wet etching or dry etching. The copper pad 4 may be formed by Chemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD). As one can know that these processes are complex and expensive. Meanwhile, the connection of copper and aluminum is not so firm and stable. Due to difference of coefficients of thermal expansion of copper and aluminum, the interface of the two metals may split under high temperature, e. g. when the IC is working. Although there are many prior arts, for example, the U.S. Pat. No. 6,756,298, disclosing specified methods to deal with adjoining of copper and aluminum for the structure of a bonding pad of an IC, the achievement will just increase the manufacturing cost. For ICs that don't need to be made under advanced manufacturing processes, it is necessary to find a simple, effective and cheap way to form the bonding pad (copper contact) over an aluminum wiring in the ICs.